1. Field of the Invention
The present invention relates to a toner and a method of manufacturing the toner.
2. Description of the Related Art
An image forming apparatus which forms images in an electrophotographic system includes a photoreceptor, a charging section, an exposing section, a developing section, a transfer section, and a fixing section. The charging section charges a surface of the photoreceptor. The exposing section irradiates the charged surface of the photoreceptor with signal light to thereby form an electrostatic latent image corresponding to image information. The developing section supplies a toner contained in a developer to the electrostatic latent image formed on the surface of the photoreceptor so that a toner image is formed. The transfer section transfers the toner image formed on the surface of the photoreceptor to a recording medium. The fixing section fixes the transferred toner image onto the recording medium. The cleaning section cleans the surface of the photoreceptor from which the toner image has been transferred. In the image forming apparatus as described above, the electrostatic latent image is developed by using as the developer a one-component developer containing a toner or a two-component developer containing toner and carrier so that an image is formed. The toner used in the above case is formed of resin particles which are obtained in a manner that, for example, a colorant and a release agent such as wax are dispersed and granulated in binder resin serving as a matrix.
Through the electrophotographic image forming apparatus, an image having favorable image quality can be formed at high speed and low cost. This promotes the use of the electrophotographic image forming apparatus in a copier, a printer, a facsimile, or the like machine, resulting in a remarkable spread thereof in recent years. Simultaneously, the image forming apparatus has faced up to more demanding requirements. Among such requirements, particular attentions are directed to enhancement in definition and resolution, stabilization of image quality, and an increase in image forming speed, regarding an image being formed by the image forming apparatus. In order to fulfill these demands, a two-way approach is indispensable in view of both the image forming process and the developer.
Regarding the enhancement in definition and resolution of the image, the reduction in diameter of toner particles is one of problems to be solved from the aspect of the developer. This is based on the perspective such that it is important to authentically reproduce the electrostatic latent image. As a method of manufacturing the diameter-reduced toner particles, the emulsion aggregation method is known, for example. In the emulsion aggregation method, coloring resin particles containing a binder resin, a colorant, a release agent, and the like ingredient are generated in water and then aggregated to thereby manufacture toner particles.
As the diameter-reduced toner particles manufactured by the emulsion aggregation method, there is a toner in form of aggregated particles of coloring resin particles made of polyester which has a glass transition temperature of 58° C. to 70° C., a softening temperature of 80° C. to 130° C., a number average molecular weight of 2,000 to 10,000, a ratio of the weight number average molecular weight to the number average molecular weight ranging from 2 to 10, and an acid number of 3.8 to 30 mg KOH/g. In the toner, the coloring resin particles have a volume average particle diameter of 0.1 μm to 1 μm, and the aggregated particles have a volume average particle diameter of 4 μm to 10 μm and a shape factor of 120 to 160 (refer to Japanese Examined Patent Publication JP-B2 3577390, for example).
In accordance with the method of manufacturing a toner disclosed in JP-B2 3577390, the toner is manufactured as follows. First of all, toner raw materials including a binder resin, a colorant, and wax are put and thus dispersed in a water-based medium which is obtained by adding a dispersion stabilizer to water, thereby producing in the water-based medium coloring resin particles having a volume average particle diameter of 0.1 μm to 1 μm. Subsequently, to the water-based medium where the coloring resin particles exist, an aggregating agent is added so as to aggregate the coloring resin particles which are then treated with heat. The water-based medium is thereafter removed, resulting in a toner which is formed of aggregated coloring resin particles.
In the method of manufacturing a toner as mentioned above, however, even when the toner raw materials are put and thus dispersed in the water-based medium, poor compatibility between the binder resin and the wax causes particles of a resin, particles of a colorant, and particles of wax to be formed respectively in the water-based medium, instead of forming the coloring resin particles in which the colorant and the wax are dispersed in the binder resin. The toner formed by aggregating those particles will have the particles of pigments and the particles of wax exposed on a surface of the toner. The particles of wax exposed on the surface cause the preservation stability to be decreased. Further, desorption of the wax particles from the toner causes the anti-offset property to be decreased. Moreover, the exposure of the colorant particles causes the toner to exhibit nonuniform charging performance.
In view of the foregoing, there is proposed a method in which fine resin particles and a charge control agent are further attached to toner particles formed of aggregated coloring resin particles, to thus form an encapsulated toner (refer to Japanese Examined Patent Publication JP-B2 3724309, for example). In the method disclosed in JP-B2 3724309, the coloring resin particles are aggregated and then heated and fused to thereby form toner particles. This aggregate of coloring resin particles will have aggregated particles of respective components, and particles of wax will be exposed on a surface of the toner, just as in the case of toner disclosed in JP-B2 3577390. In the case of JP-B2 3724309, the toner particles formed of aggregate of coloring resin particles as described above are treated with heat, pH adjustment, salt addition, and the like process so that the fine resin particles are attached to the toner particles, thus forming the encapsulated toner. The formation of encapsulated toner through attachment of the fine resin particles to the toner particles will prevent the particles of wax from being exposed, thus resulting in enhancement in the preservation stability and the anti-offset property.
In the method of manufacturing an encapsulated toner disclosed in JP-B2 3724309, however, in order to make fine resin particles be attached to the toner particles, it is necessary to treat the toner particles with heat, pH adjustment, salt addition, and the like process, thus requiring complicated operations. Consequently, there has been a demand for a toner which can be manufactured in a simple production method and which is excellent in the preservation stability and the anti-offset property.